Due to environmental issues typified by global warming, and resource issues such as the depletion of oil resources and anxiety over nuclear power resources, energy conservation has been increasingly important.
Under such circumstances, the automobile fields, for example, have been making remarkable progress in hybrid vehicles and electric vehicles that contribute to energy conservation.
Further, in the household appliance fields, there is an increasing demand for highly efficient air conditioners and refrigerators that consume less electric power.
These products commonly use motors, and hence, these motors are increasingly required to have improved efficiency.
The motors in these products have been miniaturized in response to the need for miniaturization and weight reduction, and further are designed to rotate at high speeds to meet the need for outputting sufficient power.
In order to reduce increasing losses occurring from high rotational speed and the resulting heat occurring in the devices, cores of the motors are required to be formed by a non-oriented electrical steel sheet having reduced high-frequency iron loss.
Further, these motors need to generate high torque, and there is a demand for the non-oriented electrical steel sheet to have increased saturation magnetic flux density: Bs, especially at the time of motor acceleration.
Since the eddy current loss accounts for a large portion of the iron loss in the high-frequency iron loss, the iron loss can be reduced by increasing the resistivity of the non-oriented electrical steel sheet, as described, for example, in Patent Document 1.
However, alloying, which is necessary to increase the resistivity, brings about a problem of a reduction in the saturation magnetic flux density Bs.
Further, alloying makes the steel sheet significantly brittle, which has a large adverse effect on the productivity.
In particular, if the amount of Si exceeds 3%, the reduction in Bs and brittleness of the steel sheet become notable, which makes it extremely difficult to achieve all the desired magnetic properties and productivity.
In Patent Document 1, the amount of Si+Al is controlled to be less than or equal to 4.5%. However, this control is not sufficient enough to prevent the steel sheet from becoming brittle. Further, Patent Document 1 does not take into consideration the effect of Mn, which is the main point of the present invention.
Yet further, Patent Document 1 does not evaluate Bs, and hence, favorable magnetic property cannot be necessarily obtained.
Patent Document 2 describes making the relationship between resistivity and Bs constant. However, Patent Document 2 is not intended to obtain high torque, and cannot prevent the steel sheet from becoming brittle.
Further, Patent Document 2 is not directed at improving iron loss at high frequencies, and does not take into consideration brittleness of a steel sheet having the amount of Si exceeding 3.0% or improvement in the iron loss of the steel sheet. Thus, favorable magnetic properties cannot be necessarily obtained.